A rubber-metal composite body constituted of a rubber composition and a metal material embedded in the rubber composition is used in a rubber article requiring in particular high strength such as a tire, an industrial belt, a rubber crawler and the like in order to reinforce rubber to improve strength and durability thereof. It is necessary to bond the metal material to the rubber composition stably and firmly to cause a resulting rubber-metal composite body to have a good reinforcing effect, in this regard.
It is widely practiced to carry out what is called “direct vulcanization bonding” of a metal material and a rubber composition, i.e. to coat a metal body such as steel cords each plated with zinc, brass or the like, with a rubber composition containing sulfur, and bond the metal body and the rubber composition to each other simultaneously with vulcanization of the rubber composition, in order to realize good adhesion between the metal material and the rubber composition. For example, PTL 1 discloses that it is possible to obtain a steel cord-rubber composite body having good initial adhesive properties and heat resistant adhesive properties between steel cords and a rubber composition by immersing brass-plated steel cords having specific contents of phosphor and metals such as zinc on the outermost surface thereof in a solution containing a metal salt with pH in the range of 6.1 to 8.0 and then coating the steel cords with a rubber composition and subjecting the resulting composite body to vulcanization. However, the adhesive properties of the steel cord-rubber composite body of PTL 1 still need to be improved in terms of increases in recent years in physical and thermal load applied to such rubber articles as described above.